Electrical communication system



June 5, 1934. P. o. FARNHAM 1,961,140

ELECTRICAL COMMUNICATION SYSTEM Filed June 8, 1929 1% i P H 3 C t 2 L, 5 1! ll 0 \'WMA-\9 Q.Q&QJHC RC LC A2 1.0 o h? I gvmnto c a, g a

Sl m anew Patented June 5,1934 H I UNITED STATES PATENT OFFICE ELECTRICAL COMMUNICATION SYSTEM Paul 0. Farnham, Boonton, N. J., assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application June 8, 1929, Serial No. 369,287

' 4 Claims. (01. 250-20) This invention relates to an electrical. comdrawing since the particular receiver construction munication system and particularly to an input is not an essential feature of the invention. The system for use with a radio frequency amplifier. impedance L across which the tube 1 is con- In designing a radio receiver for operation over nected may be serially connected in the input '5 a band of frequencies, the engineer is confronted system, as shown in Fig. 1, or may be coupled 60 with the fact that the stage gain-wave length in the conventional manner to a coil in the input curve falls off rapidly with increasing wave length. system. In addition to this varying amplification which In addition to the coupling impedance 11101, the characterizes the usual stage or stages employing circuit between antenna A and ground G inlO tuned transformer or impedance coupling, a fureludes the inductance Le which is serially con- 5 ther variation, and in the same sense, arises nected between the antenna, and the coupling im- Within the input system which precedes the first pedance. The inductance Le is so chosen that, amplifier. While it is possible to construct amwith its distributed capacity or with added caplifier stages having a flattened gain-wave length pacity Cc, it resonates at a wave length below the characteristic, it will be'apparent that the design operating range of the receiver. 70

of an amplifier may be materially simplified if For a broadcast receiver the impedance LcCc the input system is given such properties that, may be a radio frequency choke of approximately for a given signal strength in the antennapthe i200 microhenries, the geometry of the choke voltage impressed upon the first amplifier, inbeing such that it is resonant at about 180 mecreases with increasing wave length. ters. c 75 In considering the operation of an input sys The electrical equivalent of the input system tem, reference is usually made to the step-up of is illustrated in Fig. 2,111 which the antenna voltthe system, which step-up is given by the ratio age E0 is impressed across the series circuit which of the first stage input voltage E to the voltage includes the impedance Z0 of the collecting de- E0 induced in the antenna. vice, the impedance Z'c of the added impedance 0 An object of the present invention is, to proor choke, and the impedance Zi of the coupling vide an input system which operates to flatten, circuit Each of the three impedances is composor if desired to reverse the .slope of, the curve ite, the impedance'Zo, for example, comprising which represents the Variation of step-up with resistance R0, inductance Lo and capacity Co.

Wave length. The numerical values of these three composite 7 An object is to provide an input system which impedances are given by the equations: will equalize the gain over the operating range of a receiver by reducing the step-up at the higher h frequencies. 7 I 35 A further object is 'to provide an input system which operates, throughout the frequency range the sficolld and t rcl equat ons, the terms to which an amplifier may be tuned, to give the R and R 1 a the effectlve series relstanc?s; efiective impedance from which the receiver opthe terms X c and X1 are the effectlve ,senes emtes an inductive characteh reactances, and the terms Z'c and Z1 are the cf- 7 40 More Specifically, an object is to provide an fective series impedances of the respective par- 95 input system including, between the receiver and 31191 cn'cultsan antenna, a reactive impedance resonant at a A mathematical analysis WIH ShOYV that, for a wave length below the lowest Wave length to given source voltage E0, the receiver input voltage which the receiver may be tuned E, will be a maximum when the input system is In the accompanying drawing: resonant L I goo Fig. 1 is a circuit diagram illustrative of the 1:0 invention; approximately, and that the receiver input volt- Fig. 2 is a diagram of the equivalent electrical age E will be a minimum at the resonant frecircuit; and quency of the added impedance ZE.

- Fig. 3 is a representation of curves showing the It is to be noted that this cutoff frequency of m5 relation between voltage step-up and wave length. input Voltage E is dependent y p n the C011- In the drawing, the numeral 1 identifies the stants of the choke Z0, and is independent of the radio frequency amplifier tube which is fed from antenna constants. This feature is particularly the input system, the remaining tubes and tube important in connection with broadcast recircuits of the receiver being omitted from the ceivers since greater freedom of set designwill fll0 be possible when the operating characteristics of the receiver are not dependent upon the particular antenna with which it is associated.

The two requirements for the design of the choke impedance 20 are:

(1) Xc O, at a wave length just below the lower operating limit of the receiver, and

(2) X'c|-X0 0, for wave lengths at the upper limit of the receiver.

For broadcast reception, the first requirement may be satisfied by designing the choke for resonance at about 180 meters. To satisfy the second requirement, either the inductance Le of the choke or the antenna capacity Co is made relatively large, or both may be given comparatively large values.

The operation of the invention is graphically represented by the curve A of Fig. 3, which shows the variation of voltage step-up with wave length. The input system under measurement was a short outdoor antenna working into an inductance Lc of about 1200 microhenries and a tuned coupling impedance Ll of 200 microhenries. The data from which curve B was plotted was obtained when the choke inductance Le was removed to leave the conventional antenna inpu system.

It is to be noted that the conventional input system gave a voltage step-up at longer wave length which was about one-tenth the step-up at short wave lengths, while the input system constructed in accordance with the invention gave a step-up at 550 meters which was over six times the step-up at 200 meters.

It will be understood that the invention is not limited to the particular example which is given above, and that the choke impedance may be appropriately designed for use with receivers operating over other frequency bands, or for effecting a suppression of some particular fre-- quency or band of frequencies within the operating range of the receiver.

I claim:

1. An input system for a tuned radio frequency amplifier comprising, in series, a collecting device, an inductive element having sufficient distributed capacity to maintain said element fixedly resonant to a frequency above the frequency range of the amplifier, and an impedance across which said amplifier may be coupled, said inductive element having such constants that the ratio of signal voltage delivered to the amplifier at the low end of said range to that delivered at the high end of the range is increased from a value of the order of 0.1 to a value of the order of 6.

2. In an input system for a tuned radio frequency amplifier, the combination with a collecting device and an impedance across which the input circuit of said amplifier may be coupled, of means for decreasing the voltage step-up at higher frequencies, said means comprising an inductance having amagnitude of the order of 1200 microhenries in series with said collecting device and impedance, and said inductance having distributed capacity and being so designed as to have said capacity in shunt with said inductance and cooperating therewith to exhibit parallel resonance at a frequency adjacent to but higher than the highest frequency to be transmitted to said amplifier.

3. In combination, a source of signal energy, a stage of radio frequency amplification including an electron discharge tube provided with an input circuit, a coupling impedance connected between said input circuit and said source of signal energy, means for tuning said coupling impedance to a desired frequency in a predetermined frequency range, and an inductance element connected in series between said source and said coupling impedance, said inductance element being constructed to be fixedly resonant to a frequency above, but adjacent to, the highest frequency of said range to which said coupling impedance is tunable, and said inductance element having such constants that the ratio of signal voltage impressed on said input circuit to that impressed on said coupling impedance by said source is much greater at the low frequency end of said range than at the high frequency end.

4. In combination, a circuit constituting a source of signal energy, a stage of radio frequency amplification including an electron discharge tube provided with an input circuit, a coupling impedance coupling said input circuit with said first mentioned circuit, means for tuning said coupling impedance to a desired frequency in a predetermined frequency range, and an inductance element connected in said first mentioned circuit in series with said coupling impedance,

PAUL O. FARNHAM. 

